Stereomicroscopes are used on the one hand to manipulate objects under visual observation and on the other to make fine object details visible. The object manipulation preferably takes place under low magnification and requires good 3D reproduction. For detail recognition rapid switching to high magnifications with high resolution is desired without any change of instrument.
Greenough stereomicroscopes are described in many places in the literature, c.f. also “Optical Designs for Stereomicroscopes”, K-P. Zimmer, in International Optical Design Conference 1998, Proceedings of SPIE, Vol. 3482, pages 690-697 (1998). As stated therein, these microscopes consist of two monocular microscopes inclined towards one another. According to the prior art these two microscopes are constructed symmetrically with respect to a perpendicular on the plane of the object. Greenough stereomicroscopes provide two views of the object from different observation angles each of which is delivered to one eye, thereby providing a three-dimensional image impression. If the angle between the two directions of observation is too great, the lateral edges of the object appear out of focus.
The angle between the two microscopes, referred to as the convergence angle, is typically in the range from 10° to 12°. Since the optical axes of the two microscopes meet the object at an oblique angle, a large convergence angle is detrimental to the in-focus reproduction of the edges of the object and makes merging of the two partial images into a three-dimensional image difficult. The numerical aperture of the individual microscopes is upwardly limited by the convergence semi-angle in the conventional construction, as the lenses that define the opening cone cannot penetrate one another. The numerical aperture can in fact be increased by attachment lenses, but only if the working distance is considerably reduced. At the same time the observation angle at the object end is increased, with the negative consequences described above for the reproduction of the edges of the object.
From U.S. Pat. No. 5,603,687 an asymmetrical stereo-optical endoscope is known in which two systems of objectives with different diameters of the entry pupil are arranged parallel and side by side. The two objectives generate images of the object on a sensor surface by means of light guides. From these CCD sensors, for example, the image data are conveyed to a monitor after digital processing, i.e. they can be perceived three-dimensionally with a stereomonitor, for example. It is stated that in spite of the different diameters of the two endoscopic channels the observer perceives a stereoscopic image with a resolution and a brightness as determined by the larger-diameter channel. The second channel with a smaller diameter would serve primarily to produce a three-dimensional view.
The situation in a Greenough-type stereomicroscope of the construction described above is fundamentally different from that in an endoscope according to U.S. Pat. No. 5,603,687. First of all, the object is generally (at least additionally) viewed directly with the eyes without any digital processing beforehand. Such digital processing is or may be used if in addition documentation is to be carried out using attached cameras. Furthermore, the stereochannels or microscopes are not parallel to one another but are arranged at the specified convergence angle to one another. It is not apparent from the above-mentioned US specification how an object can be viewed visually directly with the arrangement disclosed therein. Furthermore, the imaging on one sensor surface (fixed focus) restricts the depth of focus of the image as the accommodation ability of the eyes is eliminated.
The magnification of an endoscope is dependent on the distance of the object. At high magnifications the distance of the object is generally short. In this case the area of overlap between the visual fields of the two adjacent objectives is small. Therefore in this case stereoscopic viewing, which is possible only in the area of overlap, is restricted. At low magnifications, on the other hand, the overlap is great but the numerical aperture is small, resulting in a high depth of focus. This means that the imaging quality of 3-D objects decreases only slowly with the distance from the focussing plane. This fact favours the merging of the two partial images into one three-dimensional image, particularly when the depth of the object is less than the depth of focus.
An essential component of a stereomicroscope of the kind described is an objective changer for the discrete adjustment of magnification or zoom systems (also known as objectives in the form of Vario-systems) for continuously selecting the magnification, in the case of the synchronous adjustment of magnification in the two stereochannels. Magnification systems of this kind are not common in endoscopy. Therefore, the US specification mentioned above does not discuss any variation in the scale of the imaging.
For stereoscopic observation the depth of focus is important. In contrast to the stereoendoscope described above, high-powered stereomicroscopes of the Greenough-type advantageously make use of the accommodation ability of the eyes. The magnification is varied without changing the focussing of the device. Throughout the entire range of magnifications there is no difference in the blanking of the object between the right- and left-hand partial images. The numerical aperture and hence the resolution of the stereomicroscope is matched to the magnification and avoids needless or empty magnification. At high magnifications in arrangements of this kind the depth of focus is very small, in many cases less than the depth of the object. The imaging quality of 3-D objects therefore decreases sharply with the distance from the focussing plane. It cannot therefore be assumed that the fusion of the partial images into one three-dimensional image, observed in a stereoendoscope with typically low magnification and high depth of focus, can be transferred to the circumstances which prevail in a high-powered microscope, particularly at high magnifications, when the stereoscopic channels deliver images of different resolution and depth of focus as a result of different apertures.
Another aspect which cannot be ignored is that of the brightness of the image, which varies in the above-mentioned US specification on account of the different entry pupil diameters of the endoscopic channels. Here, digital image processing has the advantage that both partial images can be displayed on the monitor with equal brightness after suitable correction. Such correction is not possible with direct visual observation such as occurs in stereomicroscopes.
From U.S. Pat. No. 3,655,259 a somatic microscope is known which is to be used as an endoscope. This microscope is designed as a stereomicroscope of the Greenough-type. The two stereochannels are at a given acceptance angle to one another and each have their own objectives, which are constructed in this case as mini lenses, rod lenses or as the end sections of a glass fibre. The problem underlying the somatic microscope in this specification is based on the fact that when two objectives are used they cannot be arranged as close together as is desired, as the objective used is a lens combination and it is impossible to use a single objective lens because of the increasing spherical aberrations, particularly when work is to be done under high magnification. The aim in the above-mentioned specification is therefore to find an arrangement which allows a narrow endoscope diameter while achieving a high magnification.
Another stereoendoscope is known from U.S. Pat. No. 4,862,873, which has two channels arranged parallel to one another, one of the channels being used for illumination while the other is used for observation. To produce a stereoscopic image impression the two channels are switched 30 times per second, for example, using a motor driven prism.